FIG. 1 (Prior Art) shows a power supply 1 which converts an alternating current (AC) sinewave input supply voltage V.sub.IN received from a power source into a substantially direct current (DC) output supply voltage V.sub.OUT1. The substantially DC voltage V.sub.OUT1 can be subsequently regulated by electronic means for subsequent use by electronic circuitry. Power supply 1 comprises four diodes D1-D4 and a capacitor C1. Resistance R1 represents a load resistor.
FIG. 2 (Prior Art) illustrates operation of the power supply of FIG. 1. When node N1 is positive with respect to node NO during a first part of the sinewave of AC voltage V.sub.IN, the voltage on node N1 is positive with respect to the voltage on node N0 but is not positive with respect to the voltage on node N3 due to the fact that capacitor C1 has a charge. Diodes D2 and D4 are therefore not forward biased and no current flows into capacitor C1. This period of time is represented in FIG. 2 as time period t.sub.A. When the voltage on node N1 increases to the point that the voltage on node N1 is two forward bias diode voltage drops greater than the voltage on capacitor C1, diodes D2 and D3 conduct. Current flows from node N1, through forward biased diode D2, through capacitor C1, through forward biased diode D3, and to node N0. Capacitor C1 is therefore charged as voltage V.sub.IN increases to its peak voltage. This period of time is represented in FIG. 2 as time period t.sub.B. As the voltage V.sub.IN decreases from the peak voltage, the voltage on capacitor C1 is positive with respect to the voltage on node N1. Diode D2 is therefore reverse biased. The voltage on capacitor C1 therefore cannot discharge through the diodes. This condition remains as the voltage V.sub.IN decreases to ground potential. This period of time is represented in FIG. 2 as time period t.sub.C. During the next half cycle of the sinewave, the voltage on node N0 is positive with respect to the voltage on node N1. During the time period t.sub.D, the voltage on node N0 is not adequately high with respect to N1 to forward bias diodes D4 and D1 due to the charge on capacitor C1. During time period t.sub.E, however, the voltage on node N0 is adequately high with respect to the voltage on node N1 so that current flows through forward biased diode D4, through capacitor C1, through forward biased diode D1 and to node N1.
It is therefore seen that the current i.sub.IN1 flowing into the power supply 1 is non-sinusoidal whereas the voltage supplied to the power supply 1 is substantially sinusoidal. The power factor of the power supply, which may be defined as: ##EQU1## is therefore not unity. This is undesirable.
FIG. 3 (Prior Art) shows a power supply circuit 2, sometimes called a boost converter. If controlled appropriately, the boost converter may be made to have a power factor closer to unity (i.e., to provide power factor correction). Boost converter 2 comprises an inductor L1, a switch S1 having an associated capacitance C2, a diode D5 having a diode recovery current of magnitude i.sub.R, and an output capacitance C1. Resistor R1 represents a load resistance on the power supply.
FIG. 4 (Prior Art) shows conventional boost converter 2 controlled by a conventional control circuit 3 which modulates the on/off duty cycle of switch S1 in order to achieve a power factor close to unity. Control circuit 3 comprises a voltage divider 4, 5 for developing a sine wave reference voltage from the input voltage V.sub.IN, an X-Y multiplier circuit 6 for varying the reference voltage in response to an error voltage, output voltage error amplifier 7 for developing the error voltage by comparing the boost converter output voltage V.sub.OUT2 to a fixed reference voltage V.sub.REF, a resistor 8 for generating a voltage signal proportional to the input current of the boost converter, a high gain current error amplifier 9 for comparing the input current signal to the output signal of the multiplier circuit to produce an input current error signal, and structure 10 for converting the input current error signal into a pulse train to control the on/off duty cycle time of switch S1 in a manner to decrease or eliminate the current error.
FIG. 5 (Prior Art) is a diagram illustrative of waveforms associated with the operation of boost converter 2 under the control of control circuit 3. In operation, the voltage on node N1 with respect to the voltage on node N0 is a fullwave rectified version of the sinewave voltage V.sub.IN. During the first half period of the sinewave V.sub.IN, the voltage on node N1 follows the voltage V.sub.IN. During the second half period, however, the voltage on node N1 is rectified as illustrated in FIG. 5 by the dashed waveform 3A. During a first time period t.sub.F, switch S1 is controlled to connect node N2 to node N0. Current therefore flows from node N1, through inductor L1 to node N2, and through switch S1 to node N0. Because the voltage on node N2 is coupled to node N0 and because capacitance C1 is charged so that a positive voltage is present on node N3, diode D5 is reverse biased. No current therefore flows into capacitor C1 from node N2. The current flowing through inductor L1, however, causes energy to be stored in inductor L1. When, during a second time period t.sub.G, switch S1 is opened, node N2 is decoupled from node N0. Inductor L1 therefore causes the voltage on node N2 to increase in accordance with the relation: ##EQU2##
With the voltage on node N2 being positive with respect to the voltage on node N3, diode D5 is forward biased and current flows through diode D5 and into capacitor C1. After the energy stored in inductor L1 has been transferred to capacitor C1 through diode D5, switch S1 is closed. Node N2 is therefore again coupled to node N0, diode D5 again becomes reverse biased, and current flowing through inductor L1 to node N0 causes another quantity of energy to be stored in inductor L1. This process of closing switch S1 to store energy in inductor L1 and then opening switch S1 to move that energy from the inductor into the capacitor C1 to charge capacitor C1 is repeated multiple times throughout the period of input voltage V.sub.IN. Due to the control of control circuit 3, the current waveform i.sub.IN2 flowing into the boost converter power supply as illustrated in FIG. 5 more closely represents the sinewave of the input voltage V.sub.IN. Accordingly, the boost converter 2 has a power factor closer to unity than does the power supply circuit of FIG. 1.
The boost converter power supply of FIGS. 3 and 4, however, is somewhat inefficient due to power losses. First, charge is stored in capacitance C2 when a high voltage is on node N2 and when switch S1 is open. When switch S1 is then closed to begin to store energy in inductor L1, switch S1 couples the two terminals of charged capacitance C2 together. The energy stored in capacitance C2 is therefore dissipated and lost without being resupplied to the input or being supplied to the output of the power supply. Second, when diode D5 is commutated from being forward biased to being reverse biased when switch S1 is closed, diode recovery current flows for a short period of time. The magnitude of this diode recovery current for a state of the art power diode may be many times the magnitude of the forward current ordinarily flowing from node N2 to node N3 during normal operation of the power supply. This diode recovery current, which flows from diode D5 and through closed switch S1 to node N0, does not contribute to the storage of energy in inductor L1, does not flow into capacitor C1, and therefore also constitutes a loss of energy.